1. Field of the Invention
The present invention relates to a heat-sealable polyester composition and the production of heat-sealable polyester film having an amorphous processing window ranging from a Tg in the range of about 40xc2x0 C. to about 70xc2x0 C., to a Tcg in the range of about 70xc2x0 C. to about 150xc2x0 C. This invention specifically relates to modification of poly(ethylene terephthalate) homopolymer or copolymer by the addition of poly(trimethylene terephthalate) homopolymer or copolymer, which results in a polymer blend or copolyester with a low Tg and high Tcg, making it heat-sealable at lower temperatures, yet retaining moderate barrier properties.
2. Description of the Related Art
Poly(ethylene terephthalate), herein referred to as 2GT or PET, and copolyesters thereof (e.g., copolyesters with isophthalate (I) or cyclohexane dimethanol (CHDM) to make 2GT-I, 2G-CHDM/T or 2G-CHDM/T-I) are known to be useful for packaging goods or foods that are sensitive to flavor loss or absorbing ambient flavors and odors, i.e., flavor scalping. For example, see U.S. Pat. No. 4,578,437. These resins are also useful to provide grease resistance. In addition, these polyesters provide moderate barrier to the transmission of oxygen, carbon dioxide, and/or water vapor.
In packaging and other applications, heat-sealing is used to join thermoplastic parts. This is done by applying heat to the surfaces to be joined to soften or melt them while applying some pressure to the place where they need to be joined. Most commonly the heating is carried out by contacting the surfaces opposite those to be joined with a hot object, such as a hot bar or heating the surfaces with hot air, infra-red radiation, ultrasonic, or induction heating. The speed at which one can heat the surfaces to be joined to the proper temperature for joining often determines the speed at which one can heat-seal the surfaces. High-speed heat-sealing is important because many such operations are high-volume, continuous operations where slow heat-sealing speeds significantly increase costs.
It would be desirable to seal the polyesters described above using thermal sealing equipment at fast sealing speeds, and still achieve strong seals. This has traditionally been difficult to achieve with 2GT homopolymer or copolymer because of the high glass transition temperature (Tg) of these compositions, typically greater than about 70xc2x0 C. Amorphous (non-crystalline) polyester films or articles will not form heat seals with themselves until the temperature of the two seal-forming surfaces are raised to a range above the glass transition.
Poly(trimethylene terephthalate), herein referred to as 3GT, also referred to as poly(propylene terephthalate) or PPT, is a polyester prepared by the condensation polymerization of 1,3-propane diol and terephthalic acid. Poly(trimethylene terephthalate) may also be prepared from 1,3-propanediol and dimethylterephthalate (DMT) in a two-vessel process using tetraisopropyl titanate catalyst, Tyzor(copyright) TPT (a registered trademark of E. I. du Pont de Nemours and Company). Molten DMT is added to 1,3-propane diol and catalyst at about 185xc2x0 C. in a transesterification vessel, and the temperature is increased to 210xc2x0 C. while methanol is removed. The resulting intermediate is transferred to a polycondensation vessel where the pressure is reduced to one millibar (10.2 kg/cm2), and the temperature is increased to 255xc2x0 C. When the desired melt viscosity is reached, the pressure is increased and the polymer may be extruded, cooled, and cut into pellets.
3GT has better oxygen barrier and flavor barrier properties compared to 2GT, but has a much lower glass transition temperature, typically around 50xc2x0 C. Unfortunately 3GT does not have stable heat-sealing performance because of the low Tg and small amorphous processing window. That is, the temperature range between the Tg and Tcg (i.e., the T between the glass transition temperature and the peak temperature of crystallization from the amorphous state) is very small so the amorphous film or part tends to age or crystallize before it seals. Once a 3GT-containing film or part crystallizes it is very difficult to achieve strong seals at temperatures below the melt temperature.
U.S. Pat. No. 5,183,623 describes a process for producing transparent and heat-resistant polyester films for containers by thermoforming techniques. The sheets are produced from a melt polyester resin having at least 80 mole % ester units derived from 1,3-propanediol and terephthalic acid or ester-forming derivative thereof. The polyester resin has at least 80 mole % 3GT based on the entirety of the repeating units in the polymer chain; this equates to approximately 80% by weight 3GT. The solid resin sheet has low crystallinity, and after aging the sheet is shaped by thermoforming under specific conditions to result in a highly crystalline container
U.S. Pat. No. 5,989,665 discloses a copolyester comprising the reaction product of a glycol component comprising from about 85 to 100 mole % 1,3-propanediol, or approximately 85.8 to 100 weight % of the same. The copolyester is disclosed to be useful in packaging applications.
U.S. Pat. No. 5,627,236 discloses a multi-phase composition of a copolyester continuous phase and a low modulus discontinuous phase. The discontinuous phase may be elastomeric, non-elastomeric, crosslinked, branched or straight chain. Particularly useful as the discontinuous phase are ethylene copolymers or terpolymers. The composition provides high temperature viscosity stability and excellent long-term, heat-seal and barrier properties.
Japanese Kokai Patent Application No. HEI 10[1998]-279707 discloses a copolyester film, preferably containing 3 to 35 wt. percent of 1,3-propanediol as the glycol component. The film is heat treated at a temperature in the range of 100-180xc2x0 C. to increase the resistance against deterioration of the film with time so that the incidence of film breakage is greatly reduced. The resulting film has a rate of extension of more than 100% after storage at 40xc2x0 C. for 5 days, among other properties.
There is a need for a polymer possessing the best barrier properties of 2GT and 3GT, that can be easily heat-sealed at low sealing bar temperatures and fast sealing speeds, yet still produces seals of high strength. Especially for use in packaging, it would be preferred that such polymer produce clear parts and films.
The invention herein provides a heat-sealable polyester composition having an amorphous processing window ranging from a glass transition temperature, Tg, in the range of about 40xc2x0 C. to about 70xc2x0 C. to a peak crystallization temperature from the amorphous state, Tcg, in the range of about 70xc2x0 C. to about 150xc2x0 C.
The invention herein further provides a heat-sealable polyester composition comprising poly(ethylene terephthalate) homopolymer or copolymer and about 5% to about 95% by weight poly(trimethylene terephthalate) homopolymer or copolymer, preferably 20% to 80% by weight, and more preferably 30% to 70% by weight, based on the total weight of poly(ethylene terephthalate) and poly(trimethylene terephthalate).
Also provided is a process for lowering the Tg of poly(ethylene terephthalate) homopolymer or copolymer comprising the steps of:
(a) adding poly(trimethylene terephthalate) homopolymer or copolymer to crystallized poly(ethylene terephthalate) homopolymer or copolymer to form a blend;
(b) drying the blend by exposing the blend to a flow of at least 1 ft3/minute of dry air having a dew point less than xe2x88x925xc2x0 F., at a temperature ranging from about 120xc2x0 C. to about 130xc2x0 C., for at least about six hours or until sufficient drying has occurred;
(c) melt blending the blend to form a polymer having a lower Tg than the poly(ethylene terephthalate) of step (a).
The invention further provides a process for making a low-temperature, heat-sealable polyester film comprising the steps of:
(a) adding poly(trimethylene terephthalate) homopolymer or copolymer to crystallized poly(ethylene terephthalate) homopolymer or copolymer to form a blend;
(b) drying the blend by exposing the blend to a flow of at least 1 ft3/minute of dry air having a dew point less than xe2x88x925xc2x0 F., at a temperature ranging from about 120xc2x0 C. to about 130xc2x0 C., for at least about six hours or until sufficient drying has occurred;
(c) melt blending the blend to form a polymer having a lower Tg than the poly(ethylene terephthalate) of step (a).
The invention also provides a process for heat-sealing two thermoplastics wherein the two thermoplastic surfaces are sealed to one another by the application of heat and pressure, wherein the improvement comprises at least one of said thermoplastics comprises a polyester composition comprising a blend of poly(ethylene terephthalate) homopolymer or copolymer and poly(trimethylene terephthalate) homopolymer or copolymer, or copolymer formed from the respective monomers.
The invention also provides an article wherein two thermoplastic surfaces have been heat-sealed, wherein at least one of said thermoplastic surfaces comprises a polyester composition comprising a blend of poly(ethylene terephthalate) homopolymer or copolymer and poly(trimethylene terephthalate) homopolymer or copolymer, or a copolymer formed from the respective monomers.
The present invention also provides a method for producing a heat-sealable film or sheet comprising the steps of:
(a) extruding a mixture of two distinct polyester fractions comprising from 95 to 5 parts by weight of a poly(ethylene terephthalate) homopolymer or copolymer fraction and from 5 to 95 parts by weight of a poly(trimethylene terephthalate) homopolymer or copolymer fraction wherein said extruding is performed at a polymer residence time in an extruder and die of from 1 to 15 minutes at a temperature from 235 to 290xc2x0 C.;
(b) quenching the extruded polyester melt produced in step (a); and
(c) recovering a polyester film or sheet having an amorphous processing window ranging from a glass transition temperature, Tg, in the range of about 40xc2x0 C. to about 70xc2x0 C. to a peak crystallization temperature from the amorphous state, Tcg, in the range of about 70xc2x0 C. to about 150xc2x0 C.
In this later process the two distinct polyester fractions preferably comprise pellets of poly(ethylene terephthalate) homopolymer, pellets of poly(ethylene terephthalate) copolymer or their mixture as one distinct polyester fraction and pellets of poly(trimethylene terephthalate) homopolymer, pellets of poly(trimethylene terephthalate) copolymer, or their mixture as another distinct polyester fraction. Preferably one or more of the two distinct polyester fractions are dried prior to extruding by exposing the fraction(s) to a flow of at least 1 ft3/minute of dry air having a dew point less than xe2x88x925xc2x0 F., at a temperature ranging from about 120xc2x0 C. to about 130xc2x0 C., for at least about six hours.
According to this later method of the instant invention, it has been discovered that transesterification of the 2GT/3GT homopolymer/copolymer mixture during the extended residence time at extrusion temperature can be confirmed by NMR and advantageously correlated with the desired low temperature, heat-sealablity of the resulting polyester film. More specifically, a peak at approximately 8.038 ppm of the 1H NMR spectrum, 500 MHz and higher in tce-d2, deuterated tetrchloroethytlene, @ 120xc2x0 C., (located between major homopolymer NMR peaks associated with 2GT and 3GT) has been observed to correlate with the improved heat-sealability of the amorphous polymer film.